The invention relates to thermally removable, annealable, polymeric encapsulants and conformal coatings. More particularly, the invention relates to thermally removable, polymeric encapsulants prepared using the Diels-Alder cycloaddition reaction and their method of making.
Polymeric encapsulant materials, as both solids and foams, are used for a wide variety of applications due to their physical, mechanical, and electrical properties to give an encapsulated component structural support and protection from adverse environments. In some applications, for purposes such as recovery, analysis, and replacement, removal of the encapsulant (or conformal coating) from a component without damaging the device becomes necessary. Current techniques, such as those used for traditional epoxy and urethane foams, involve labor-intensive mechanical removal, thereby posing a substantial threat for component damage during encapsulant removal as well as increased expense for such labor intensive methodologies.
The mechanical stability of the polymeric encapsulant materials is usually associated with the fact that the polymeric material is crosslinked. Crosslinking is can be achieved in a number of ways. Typically, a polymeric or oligomeric material can have pendent groups that are linked together directly or with the help of other linking agents. The other common method of generating networks is to use two monomers with complimentary functionality and functionality greater than two per monomer. One type of functional group, A, reacts and condenses with another type of functional group, B, to form a new chemical group, the adduct. For instance, if A and B represent functionality that can form a bond, then a network can be formed from a monomer with 2 A funtionalities, (termed A.sub.2) and a monomer with greater than two B functionalities (for example three B groups in a monomer would be termed B.sub.3). An optimized stoichiometry would be N equivalents of A.sub.M and M equivalents fo B.sub.N. An important advantage of using small molecules in the network are that the depolymerized network will have increased solubility and decreased viscosity relative to a network using undepolymerizable polymer/oligomer and depolymerizable crosslinks.
Diels-Alder reactions between a diene and a dienophile, typically reversible, are known and potentially useful in forming cross-linked materials. The rate of reaction between a diene and dienophile is determined by the diene and dienophile used. Likewise the rate of the reverse reaction (for depolymerization) is also determined by the individual components and the substituents on them. Typically, upon heating, the position of the equilibirum between adduct, and dienophile/diene shifts to increase the amount of the diene and dienophile. As shown as follows, the reversible Diels-Alder reaction of a maleimide, the dienophile, with functional group R, and a furan, the diene, with functional group R', is known to proceed forward rapidly at 60.degree. C.; however, at a higher temperature, depending upon the particular reactants used, the equilibrium shifts to regenerate the maleimide and furan groups. ##STR1##
A wide variety of functional groups are tolerated by the Diels-Alder reaction. In general, any organic functional group can be used for R or R' where the functional group does not undergo a preferential Diels-Alder reaction.
Meurs (U.S. Pat. No. 5,641,856, issued on Jun. 24, 1997) describes a remoldable cross-linked resin obtained from reacting a dienophile and a 2,5-dialkylsubstituted furan. The furans are substituted at both the 2 and 5 positions to prevent unwanted side reactions that cause irreversible crosslinking; the furans may also be substituted at the 3 and 4 positions with alkyl or alkyloxy groups. Meurs utilizes polymeric furans as the diene compounds; as polymeric solids, the reaction temperatures must be sufficiently high to allow mixing of the reactants. As shown in the examples of Meurs, the reaction temperature is 150.degree. C. or higher. The remolding is carried out at a temperature above 80.degree. C., more preferably above 110C., and in particular above 140.degree. C. The higher reaction temperature required by using solids as reactant is a significant disadvantage.
Iyer and Wong (U.S. Pat. No. 5,760,337, issued on Jun. 2, 1998, and U.S. Pat. No. 5,726,391, issued on Mar. 10, 1998) describe thermally reworkable binders for semiconductor devices wherein the reworkable binders comprise a crosslinked resin produced by reacting at least one dienophile with a functionality greater than one with at least one 2,5-dialkyl substituted furan-containing polymer with a filler material. lyer and Wong ('391) also discuss that the furans are substituted at both the 2 and 5 positions to prevent unwanted side reactions that cause irreversible crosslinking. As in Meurs, polymeric furans are utilized as the diene compounds; as polymeric solids, the reaction temperatures must be sufficiently high to allow mixing of the reactants. Due to the viscosity of the polymeric furans, the reworking temperature of this system is 100.degree. C. and preferably from about 130.degree. C. to about 250.degree. C.
Thermally reversible curing system with lower curing and reworking temperatures would be useful in certain applications, such as in microelectronics applications where encapsulation at high temperatures might degrade the components. Useful also would be reactants that are liquid at or near room temperature to provide easier encapsulation processing. This also facilitates de-encapsulation.